Lawrence Berkeley National Laboratory

The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above ~1000 degrees C has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12-17 at.%Si, 8.5 at.%B, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation ("pesting"), fracture toughness and fatigue-crack propagation resistance of four such alloys, which consisted of ~21 to 38 vol.% a-Mo phase in an intermetallic matrix of Mo3Si and Mo5SiB2 (T2), were characterized at temperatures between 25 degrees and 1300 degrees C. The boron additions were found to confer superior "pest" resistance (at 400 degrees to 900 degrees C) as compared to unmodified molybdenum silicides, such as Mo5Si3. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi2, for the I/M processed microstructures with coarse distributions of the a-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa sqrt m at ambient temperatures to almost 12 MPa sqrt m at 1300 degrees C.